In modern television systems it is known to automatically control the contrast and brightness of a reproduced image. For example, a TV system with automatic contrast control to inhibit "white spot blooming" is described in U.S. Pat. No. 5,003,394 entitled DYNAMIC VIDEO SYSTEM INCLUDING AUTOMATIC CONTRAST AND "WHITE STRETCH" PROCESSING SECTIONS issued to William A. Lagoni and assigned to the assignee of this application. Another TV system which includes automatic contrast control is described in U.S. Pat. No. 5,204,748 titled BEAM CURRENT LIMITING ARRANGEMENT FOR TELEVISION SYSTEM WITH PICTURE-IN-PICTURE PROVISIONS issued to William A. Lagoni and assigned to the assignee of this application and whose teachings are incorporated herein by reference. Automatic contrast control (which is also referred to as "autopix" where "pix" is an abbreviation for "picture") prevents loss of detail sharpness in highlight (white) areas due to blooming, while permitting high contrast (and therefore subjectively bright) images when the signal peaks remain below the blooming threshold.
Automatic contrast control circuitry used in modern TV systems is shown in FIG. 1. [For purpose of clarity only those portions of a TV system deemed pertinent to a discussion of the invention are shown in FIG. 1. A more detailed description of the TV systems is presented in the references cited above.] Referring to FIG. 1, there is shown main and auxiliary video inputs 1 and 3, respectively, coupled to a picture-in-picture (PIP) processor 5 which is also controlled by a receiver control 7. PIP processor 5 provides signals (C and Y) to luminance circuit 9 and to chrominance circuit 11. The outputs of luminance and chrominance circuits 9 and 11, respectively, are applied to a matrix 10 whose outputs are red(r), blue(b), and green(g) color signals which are applied to respective inputs of contrast control section 13r, 13b, and 13g. The contrast control section (e.g., 13r, 13b and 13g) is responsive to the red (r), blue (b) and green (g) color signals and its outputs are applied to a brightness control section (e.g. 15r, 15b and 15g) whose outputs are coupled via drivers (e.g. 17r, 17b and 17g) to a picture tube (e.g. 19).
The automatic contrast control arrangement includes a combiner circuit 47 for deriving a "combined" signal (e.g., SUMY) from the ouputs of the brightness section. The combined signal (i.e., SUMY) as used herein and in the claims appended hereto is representative of the luminance component of the displayed image. The combined signal (SUMY) is then processed via a peak detector 49 and a comparator 50 whose output is fed back via a buffer 51 to the control input of the contrast control section (13r, 13b, 13g) of the TV system. The autopix loop comprising peak detector 49, comparator 50, and buffer 51 defines a feedback loop coupled between the output (terminal 14) of the combiner circuit 47 and the input control (terminal 13) of the contrast control section which determines the gain of the contrast control section.
It should be noted that the matrix 10, the contrast control section 13r, 13b and 13g, the brightness control section 15r, 15b and 15g, and the combiner circuit 47 are all formed on the same integrated circuit (IC) which may be, for example, a TA7730 available from the Toshiba Corporation. As a result of the circuitry being formed on an IC the inputs to the IC and the outputs from the IC are predefined and are not easily modified.
A problem with the TV system of the type shown in FIG. 1 is that any instability associated with the autopix feedback loop may cause disturbances (e.g. flutter) in the image displayed on the picture tube.
In fact, such a problem was noted in the operation of TV systems containing circuitry of the type shown in FIG. 1 in that there was observed intermittent fluttering of the image displayed by picture tube 19 during several different modes of operation. For example, while making use of the features available with the PIP system, fluttering was seen during the shifting, freezing, expanding and contracting of the small picture and during the expanding, shifting, freezing and zooming of the large picture.
Applicant recognized that the problem was not due to a suspected instability in the autopix feedback loop. Rather, Applicant recognized that the combined output signal (SUMY) was not "blanked" during vertical retrace and contains extraneous and unwanted signals which are injected into the autopix feedback loop during vertical blanking (retrace) periods and that it is these signals which caused the observed flicker.
Applicant also recognized that since the the combined output signal (SUMY) is produced within the IC, the combined signal produced on the chip can not be easily or readily altered without modification of the IC.